I don't know that much about aruncus, I always admired it but only got my own this spring. My guess would be that it has seeded itself for you. I bought seeds from Gardens North and found it pretty easy to start under lights indoors. My seedlings appear to be thriving. I think there's a different plant that's also called goats beard so that might be the confusion on google searches.

You know, i think i might have had the wrong name all along. I think mine is actually Persicaria polymorpha. I just googled the two and it looks as if that's what my giant plant is, and the smaller ones next to it might be aruncus - only half the size. None of them have ever reseeded for me, not that i've noticed, anyway.

Makes sense to me. I will go look that one up. I had the name saved somewhere but of course I can never find it when it's blooming. I love this plant. I'm thinking mine came from a root. I don't think it reseeds... I'm excited either way with the second plant...